Polish Journal of Environmental Studies Vol. 12, No. 5 (2003), 629-633
Letter to Editor
Distribution and Forms of Heavy Metals
in Some Agricultural Soils
C. Aydinalp1*, S. Marinova2
Uludag University, Faculty of Agriculture, Department of Soil Science, 16059 Bursa, Turkey
2
Nicola Poushkarov Institute of Soil Science and Agroecology, Sofia, Bulgaria
1
Received: 23 January, 2003
Accepted: 21 March, 2003
Abstract
Distribution and availability of heavy metals to plants is important when assessing the environmental
quality of an area. The objectives of this study conducted in 2002 were: a) determine the levels of the heavy
metals cadmium (Cd), copper (Cu), lead (Pb), manganese (Mn), nickel (Ni) and zinc (Zn) in the agricultural
soils of the Bursa plain so that the degree of pollution could be ascertained, b) identify the various heavy
metal forms present in soils using a fractionation scheme based on sequential extraction, and c) to find
possible dependence on soil physicochemical properties. Total heavy metal content of the soils studied was
generally higher than the levels reported in literature for similar soils, suggesting some degree of pollution
with heavy metals. The exchangeable forms of the heavy metals, however, were very low, indicating that
under present conditions, the availability of heavy metals to plants is at a minimum.
Keywords: heavy metals, agricultural soil, pollution, extraction methods
Introduction
The loading of ecosystems with heavy metals can be
due to excessive fertilizer and pesticide use, irrigation,
atmospheric deposition, and pollution by waste materials. In natural ecosystems, and especially in wetlands,
watershed management plays an important role, determining not only the degree of plant uptake and soil retention of the heavy metals but also the extent to which
they are leached into aquifers. A precise knowledge of
heavy metals concentrations, the forms in which they
are found, their dependence on soil physicochemical
properties provide a basis for careful soil management
which will limit, as far as possible, the negative impact of heavy metals on the ecosystem. A knowledge
of present heavy metal pollution levels within the
watershed would be a starting point in estimating the
consequences of poor watershed management regimes
*Corresponding author; e-mail: [email protected]
which may mobilize previously unavailable forms of
the heavy metals and lead to their incorporation into
the food chain.
Heavy metals in soil may be found in one or more of
the following forms:
a) dissolved (in soil solution),
b) exchangeable (in organic and inorganic components),
c) as structural components of the lattices of soil minerals,
d) as insoluble precipitates with other soil components.
The first two forms are available to the plants while
the other two are potentially available in the longer term.
Understanding the mechanisms by which a heavy
metal element changes from one form to another and the
speed at which it does so, is imperfect but improving. In
general, the concentration of an element in the soil solution is believed to depend on the equilibrium between the
soil solution and solid phase, with pH playing the decisive
role [1]. The soil’s ability to immobilize heavy metals increases with rising pH and peaks under mildly alkaline
conditions. Heavy metal mobility is related to their im-
630
mobilization in the solid phase. Fuller [2], in discussing
the relatively high mobility of heavy metals with regard to
pH, considered that in acid soils (pH 4.2-6.6) the elements
Cd, Ni, and Zn are highly mobile, Cr is moderately mobile, and Cu and Pb practically immobile, and in neutral to
alkaline (pH 6.7-7.8), Cr is highly mobile, Cd and Zn are
moderately mobile and Ni is immobile.
Apart from pH, other soil properties, such as cation
exchange capacity (CEC), organic matter content, quantity and type of clay minerals, the content of the oxides of
iron (Fe), aluminum (Al), and manganese (Mn), and the
redox potential determine the soil’s ability to retain and
immobilize heavy metals. When this ability is exceeded,
the quantities of heavy metals available to plants increase,
resulting in the appearance of toxicity phenomena.
Heavy metals tend to form complexes with organic
matter in the soil (humic and fulvic acids), which are
different for each metal [3]. Organic matter plays an important role not only in forming complexes, but also in retaining heavy metals in an exchangeable form. These two
properties affect each heavy metal differently. For example, Cu is bound and rendered unavailable chiefly through
the formation of complexes [4], while Cd is retained in an
exchangeable form and is more readily available [5].
The CEC of a soil depends upon its organic matter
content and clay type and content. In general, the higher
the CEC the greater the ability to retain heavy metals.
The type and quantity of clay determines the CEC, which
increases with clay content, particularly when it contains
a high proportion of 2:1 lattice-type minerals (e.g., montmorillonite). The specific soil surface is also closely related to clay content and type. Korte et al. [6] reported that
the soil’s ability to retain heavy metals is more closely
tied to the specific surface than to the soil CEC.
In cases of soil pollution by heavy metals, it is important to identify the available and unavailable forms of the
heavy metals to ensure that the soil is managed in such a
way as to prevent the unavailable forms from becoming
available. The most common and simple way to identify
the forms in which heavy metals are found in soils is to
use sequential extraction in which components loosely
held by the soil are extracted first, followed by those
more tightly bonded.
The various forms of the heavy metals thus sequentially extracted can be classified as dissolved, exchangeable, organically-bound, or bound to oxides. As Beckett
[7] pointed out, the fractionation of heavy metals into
various forms on the basis of sequential extraction is only
operational and cannot indicate a specific mechanism,
since it is by no means certain that a given extract does
not contain smaller quantities of another form, nor that the
extractant would dissolve similar forms (e.g., carbonates)
of different metals. Nevertheless, it is useful to attribute
a specific fraction to each extractant. Thus, neutral salts
like potassium nitrate (KNO3) are assumed to take up
exchangeable forms of heavy metals, sodium hydroxide
(NaOH) organically-bound forms, Na2EDTA forms associated with carbonate salts, while strong acids like nitric
Aydinalp C., Marinova S.
acid (HNO3) take up chiefly that fraction which is structural component of mineral lattices and surfaces.
Beckett [7] in his extensive review suggested that is
preferable to classify the metal by its extract, e.g., EDTA
extract, and to describe the experimental method exactly.
However, this approach would ignore the purpose of sequential extraction, which is to investigate the chemistry
of various forms of heavy metals in the soil. It would be
preferable to find extractants that will actually distinguish
between the various metal forms on the basis of their
chemistry. Earlier studies in the wider area of the Bursa
plain have shown by sequential extraction that heavy metals exist in the soils at various sites [8, 9].
The objectives of this research are to conduct a survey
based on the total heavy metal content required for monitoring future pollution trends, and then identify what common forms exist in order to asses the availability of the
heavy metals to plant in the agricultural soils of the Bursa
plain. An attempt was also made to correlate heavy metal
concentrations with other easily measurable physical and
chemical properties of the soils.
Experimental Procedure
The study area (Figure 1) is located 10 km east of
Bursa city between 40°13′ - 40°14′ N latitudes and 29°10′
- 29°20′ E longitudes in Turkey, and is a very recent alluvium.
Soil sampling was done on a grid basis with each
square approximately 250x250 m. At each junction point
of the grid, five surface subsamples (0-25 cm) were taken
with an auger-type sampler within a radius of 5 m and
Fig. 1. The location of the research area in the Bursa province,
Turkey.
631
Distribution and Forms of Heavy Metals in...
estimate the quantity of the remaining extractant and to
calculate the amount of heavy metal carried over to the
next step. Heavy metal concentrations in all extracts were
determined by atomic absorption spectrometry. Statistical
analyses were performed using the correlation procedure
(Pearson test) and the generalized linear model procedure.
Results and Discussion
Fig. 2. Range and mean value of total heavy metal concentration
for studied soils.
mixed to a composite sample. The composite samples
were air dried, crushed lightly, and then passed through
a 2-mm sieve. All subsequent analyses were performed
on the <2-mm fraction using standard methods. Particle
size distribution determined by the hydrometer method
[10], pH in a 1:2 soil:water ratio [11], organic matter with
the wet oxidation method [12], and equivalent calcium
carbonate volumetrically [13]. Electrical conductivity
was measured in a saturation extract. The analyses were
performed on 25 soil samples from this area, resulting
in the measurement of some soil properties. Thus, soil
pH ranged from 7.6 to 8.6, CaCO3 from 3.0 to 65%, EC
from 1 to 120 mS cm-l, organic matter content from 0.5 to
5.0%, and clay content ranged between 3.8-5.7%. X-ray
analyses showed that the predominant clay minerals were
micas and chlorites.
Sequential extraction was carried out according to
Emmerich et al. [14]. After removing the saturation
extract using a vacuum and a Buchner funnel, the soil
sample was leached two to three times with 15-20 ml of
deionized water to remove the soluble salts. Two grams
of dry soil (on a 105°C basis) were placed in preweighed
centrifuge tube (three replicates) and the following sequence was used.
1. Add 25 ml 0.5M KNO3, shake for 16 h, centrifuge and
filter the supernatant liquid.
2. Add 25 ml of deionized water, shake for 21 h, centrifuge and filter the supernatant liquid. Extraction with
water on a number of samples showed that the concentration of heavy metals in the extract was below the
sensitivity limit of the atomic absorption spectrometer, and therefore this step was omitted in the rest of
the samples.
3. Add 25 ml 0.5M NaOH, shake for 16 h, centrifuge and
filter.
4. Add 25 ml 0.05N Na2EDTA, shake 6 h, centrifuge and
filter.
5. Add 23 ml 4M HNO3 and heat for 16 h in a water bath
at 70-80 °C. After cooling, the solution was filtered.
After each step, the centrifuge tube was weighed to
The range and mean values of the total quantity of
each heavy metals extracted, calculated as sum of all fractions for all sampling sites are given in Figure 2.
In this figure, it is apparent that while the concentration for some metals (Cd) is very low, for some others
(Mn and Zn), they are quite high. This can be attributed to
pollution sources existing in the surrounding area. Overall, the range of values indicates uniform spatial distribution for most of the heavy metals. For Mn, the presence
of very high values can be attributed in the presence of
Mn-oxides concentrations (field observations) due to locally reduced conditions.
The range and mean values of total heavy metal content obtained from all sampling sites along with comparative values from several sources found in the literature
[15, 16, 17, 18, 19] for non-polluted soils are shown in
Table 1. The mean and the lower limit values are higher
while the upper limit values are lower than the comparable values given by the above identified author, indicating some degree of pollution, probably due to the effects
of industrial activities in this area [9]. Also, Adriano [20]
reported that alluvial soils showed a mean Cd content of
1.5 mg kg-l (range 0.1-6.0 mg kg-l) and it is known that
chlorite minerals sometimes contain Cr and Ni.
The absolute quantities of each heavy metal extracted
with different extractants are given in Figure 3, with each
heavy metal fraction expressed as a percentage of the total
heavy metal quantity. As expected, in agreement with the
findings of Emmerich et al. [14] and McGrath and Ce-
Fig. 3. Percentage of extracted metals in using sequential extraction.
632
Aydinalp C., Marinova S.
Table 1. Average and range values of total heavy metal content (g kg-l) of all sampling sites and related background values from different sources.
Metal
Cd
Cr
Cu
Mn
Ni
Pb
Zn
Values of sites
Bowen, [15]
Background values from different sources
Shacklette&
Vinogradov,
Rose et al.
Boerngen, [16]
[17]
[18]
Mitchell,
[19]
Min.
0.1
0.01
---
---
---
---
Max.
8.7
2.00
---
---
---
---
Aver.
2
0.35
---
---
---
---
Min.
42.0
5
1
---
---
5
Max.
329.2
1500
2000
---
---
3000
Aver.
124.5
70
54
200
6.3
---
Min.
14.2
2
1
---
---
10
Max.
68.9
250
700
---
---
100
Aver.
40
30
25
20
15
---
Min.
587.9
20
2
---
---
200
Max.
3112
10000
7000
---
---
5000
Aver.
1667.1
1000
550
850
320
---
Min.
54.6
2
5
---
---
10
Max.
378.0
750
700
---
---
800
Aver.
157.8
50
19
40
17
---
Min.
33.4
2
10
---
---
20
Max.
163.3
300
700
---
---
8
Aver.
80.9
35
19
---
17
---
Min.
187.9
1
5
---
---
---
Max.
1087.0
900
2900
---
---
---
Aver.
476.7
90
60
50
36
---
garra [21] who used the same extraction procedures, the
quantity of heavy metals extracted with KNO3, expect for
Mn was less than 0.1% of the total extracted. Manganese
extracted with KNO3 ranged between 0.01 to 12.5 mg kg-l
with a mean value of 1.8 mg kg-l and its presence can be
attributed to the Mn-oxides (field observations) that exist
in these soils, which are dissolved during periodic flooding of the soils.
Sodium hydroxide extracted the highest quantity of
Cu, 26% of the total, with a mean value of 1.08 mg kg-l
(range 2.8-26.0 mg kg-l). A statistically significant relationship (r=0.7, p<0.001) between NaOH-Cu and soil organic matter content suggests that NaOH-extracted Cu is
associated with organic matter. This is in agreement with
the findings of Stevenson [3], who reported that 20-50%
of the Cu in soils occurs in the form of complexes with
organic matter.
Sodium hydroxide-extractable Zn accounted for 5.2%
of the total with a mean value of 29.4 mg kg-l and a range
from 1.3 to 189 mg kg-l. McGrath and Cegarra [21] re-
ported an average of 2.5% for NaOH-extracted Zn. Pb,
Cr and Cd were <0.1% while Mn was in the same order
with KNO3-Mn (0.1% of the total) and Ni averaged 2.8%
of the total.
Pb showed the highest (59% of the total) quantities
extracted with Na2EDTA with a mean value of 49 mg
kg-l and range 7.7 to 128 mg kg-l. Archer and Hodgson
[22] reported a similar percentage of Na2EDTA-Pb
when Na2EDTA was used as a single extractant for soil
Pb. Zn and Mn extracted with Na2EDTA were 27.0%
and 24.5 % of the total, while Cu, Ni, and Cr were
18.1%, 9.6%, and 4.6% respectively. Cd showed very
large spatial variability with values ranging from 0.01
to 3.7 mg kg-l.
Nitric acid extracted the major fractions of all the
heavy metals except for Pb. For example, HNO3-extractable Cr was 92%, Ni 87.4%, Mn 75.2%, Zn 67.5%, Cd
59.2%, Cu 55.5% and Pb 40% of the total. The results
were expected since HNO3 is the strongest of the reagents
used, which dissolves the finer soil particles, and there-
Distribution and Forms of Heavy Metals in...
fore, heavy metals that exist as structural components of
the soil minerals are dissolved and measured.
Statistically significant relationships were found
between the structural components of the various heavy
metals (HNO3 extracted) and HNO3-extracted Mn. For
example, Mn-Cu: r=0.893, p<0.001, Mn-Zn: r=0.677,
p<0.001; Mn-Cr: r=0.598, p<0.001, and Mn-Pb: r=0.89,
p<0.001. The close relationships indicate retention of Cu,
Zn, Cr, and Pb on Mn-oxides.
Conclusion
The total heavy metal contents of the investigated
soils were generally higher than the comparative levels
reported in the literature for similar soils. The exchangeable forms of these metals were very low, indicating that
under the present conditions, the availability of these
metals to plants would be minimal. The largest proportion of the metals occurred in forms that are considered
immobile, being structural constituents of inorganic minerals or carbonate compounds. Cu and Zn were present
in appreciable quantities as organically-based forms. The
spatial distribution of Cu and Zn correlated to the soil organic matter distribution. There were indications that the
immobile fraction of the heavy metals was adsorbed onto
Mn-oxides.
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